Growth & Development

Dying to grow: Programmed Cell Death key to an epiphytic orchid’s root development

Li and colleagues combined the use of transmission electron microscopy, x-ray microtomography, and transcriptome methods to characterize the major anatomical and molecular changes that occur during the development and death of velamen radicum cells of Cymbidium tracyanum, a typical epiphytic orchid.

Death is a part of life, but in the case of the epiphytic orchid Cymbidium tracyanum death is a part of growth too. A recent study by Li and colleagues found that Programmed Cell Death (PCD) plays a key role in the development of the orchid’s roots, specifically the root velamen radicum. The velamen radicum is a spongy tissue which covers the outer surface of the epidermis of epiphytic plant roots. Oh, and this spongy tissue is made up of multiple layers of dead cells. 

But wait, before you even think about exfoliating away those dead velamen radicum cells, remember that this spongy tissue helps the plant absorb water and may support nutrient uptake. According to Li and colleagues, the development of the velamen radicum “is an important characteristic for water uptake and retention in some plant families, particularly in epiphytic orchids, for survival under water-limited environments”.

Cymbidium tracyanum is an epiphytic orchid, which grows on tree trunks in the subtropical forests of southwestern China at altitudes ranging from 1200 to 2000 m above sea level. Image: Averater / Wikipedia

To gain a better understanding of the PCD process, Li and colleagues characterised the major anatomical and molecular changes that occur during the development and death of velamen radicum cells in Cymbidium tracyanum roots. Using a combination of transmission electron microscopy, x-ray microtomography, and transcriptome methods, they found three tightly linked steps involved in the development of the root velamen radicum. These three steps broadly match the developmental process that has previously been observed in xylem vessels and fibres.

Step 1: When PCD is initiated, the cell and vacuole size of the root velamen radicum both increase, and several genes involved in brassinosteroid and ethylene pathways are up-regulated. These two plant hormones (brassinosteroid and ethylene) are also involved in the initiation of cell death in xylem tissues.

Step 2: The secondary cell wall is formed through a series of significant anatomical changes, including DNA degradation, thinning of the cytoplasm, organelles decrease, vacuole rupture, and finally cell wall thickening. The authors also found “changes in the expression of genes related to the biosynthesis of cellulose and lignin, which are instrumental in the formation of secondary cell walls”.

Step 3: In the final stage of PCD, the cell dies and destroys itself through the action of its own enzymes (also known as autolysis). Li and colleagues write that “cytoplasmic degradation by autophagy is likely to be fundamental for plant PCD” and “the final process of autolysis occurred gradually but exponentially from the outside to the inside of the velamen radicum.”

Caption: Molecular controls and anatomical changes involved in the development of the root velamen radicum. Source: Li et al (2020).

Now that the process of PCD in root velamen radicum is better understood, the authors suggest that “further research, based on investigation of how the process of developmental PCD changes with environmental conditions, is still required to understand the mechanisms underlying ecological adaptations of epiphytic orchids.”

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